This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Ribonucleotide reductases (RNRs) use radical-based chemistry to catalyze an essential step in DNA biosynthesis and repair, the conversion of ribonucleotides to deoxyribonucleotides, and are important targets for anti-cancer therapy. Human RNR and other class I RNRs are composed of two types of subunits, the reductase subunit, [unreadable], and the radical-generating [unreadable]. E. coli RNR has been extensively studied and serves as a model system for class I RNRs. Despite its importance for cancer research, no intact complex of class I RNRs has ever been visualized by structural methods. As a result, a symmetrical docking model for the E. coli complex that has been proposed based on the individual crystal structures of the subunits has not been verified. Moreover, although the active complex of class I RNRs has long been thought to exist as [unreadable]2[unreadable]2, the oligomerization state of class I RNRs has been the subject of some recent debate. Efforts to understand the oligomerization states of RNR have been hindered by the complexity of the enzyme as well as by the limitations in techniques that have been used thus far. Here, we propose to use a structural study of E. coli and human RNRs using small-angle X-ray scattering (SAXS).